1. Field of the Invention
The present invention relates to a spark plug for an internal combustion engine, and also to an internal combustion engine igniting system using the same.
2. Description of the Related Art
A semicreepage discharge type spark plug for use in an internal combustion engine and having improved resistance to contamination has been known. This type of spark plug, like most spark plugs, has a center electrode, an insulator that surrounds the center electrode, and a ground electrode. The ground electrode has a firing surface formed on a tip end side and opposing a side face of the center electrode. The tip end portion of the insulator is located between the center electrode and the firing surface of the ground electrode, so that a spark discharge occurs along the surface of the tip end portion of the insulator.
When an aerial discharge type, as opposed to the above-mentioned semicreepage discharge type, spark plug is used for a long time in a low-temperature environment in which the electrode temperature is 450.degree. C. or lower, a so-called "smolder" or "dry and wet carbon fouling" state arises. In such a state, the surface of the insulator becomes covered with an electrically conductive contamination material (e.g., carbon), which can easily result in an operation failure.
By contrast, in the above-mentioned semicreepage discharge type spark plug, a spark discharge occurs such that the discharge creeps over the surface of the insulator, and hence such a contamination material always burns off. Therefore, a spark plug of the semicreepage discharge type is superior in resistance to contamination than that of the aerial discharge type.
Referring to FIG. 6, in the semicreepage discharge type spark plug, voltage application is performed so that the center electrode 2 is negative and the ground electrode 4 is positive, and a spark frequently creeps along the surface of the insulator 3. In such a situation, a channeling phenomenon, in which the surface of the insulator 3 is cut away in a channel-like form, readily occurs. When channeling advances, problems such as impaired heat resistance of the spark plug and lowered reliability readily occur. Due to the recent advancement of engine output, a spark plug having higher durability and little or no channeling has been desired.